There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as radio communication, satellites, portable computers, and electric vehicles to name but a few. There have concurrently been recent efforts to develop high energy, cost-effective batteries and/or electrochemical capacitors having improved performance characteristics.
Rechargeable or secondary cells are more desirable than primary (non-rechargeable) cells since the associated chemical reactions which take place at the positive and negative electrodes of the battery are reversible. Electrodes for secondary cells are capable of being regenerated (i.e., recharged) many times by the application of an electrical charge thereto. Numerous advanced electrode systems have been developed for storing electrical charge. Concurrently, much effort has been dedicated to the development of electrolytes capable of enhancing the capabilities of electrochemical cells.
Heretofore, electrolytes have been either liquid electrolytes as found in conventional wet cell batteries or solid films as are available in newer, more advanced battery systems. Each of these types of systems has advantages, though they have inherent limitations which make them unsuitable for particular applications. For example, solvent free solid polymer electrolytes have heretofore had vastly inferior properties such as low ionic conductivity. Glassy inorganic electrolytes have suffered from brittleness, interfacial resistance and narrow voltage limits. Specifically, conventional solid electrolytes have ionic conductivities in the range of 10.sup.-5 S/cm, whereas acceptable ionic conductivity is typically in the range to 10.sup.-3 S/cm. Good ionic conductivity is necessary to ensure a battery system capable of delivering usable amounts of power for a given application. Good ionic conductivity is necessary for the high rate operation demanded by, for example, cellular phones, power tools, and portable computers. Accordingly, solid electrolytes are not yet adequate for many high performance batteries.
With respect to liquid electrolytes, many such systems have been known for many years but have yet to yield acceptable performance results in newer battery systems, such as lithium polymer and lithium ion battery systems. Molten organic salts, because of their generally high ionic conductivities, would be preferred electrolytes in lithium polymer and lithium ion electrochemical cells. However, most known molten salt electrolytes are inorganic and their use is effectively limited to high temperature applications because of melting points that typically exceed approximately 450.degree. C. Lithium salts in particular have high melting points; however, lithium salts are potentially the most interesting for use as electrolytes in lithium polymer and lithium ion electrochemical cells. Further, where typical liquid electrolytes are concerned, evaporation and permeation pose significant concerns. However, pure molten salt materials would not pose these problems.
Accordingly, there exists a need to provide a stable, molten, organic lithium salt electrolyte which has a melting point that allows for use in the liquid phase at ambient temperatures. These materials should be relatively easy to synthesize, possess high ionic conductivity, and be stable for long term use in electrochemical cells.